The present invention relates to a method of discriminating a sheet and, more particularly, to a method of discriminating the defect of a plurality of bank notes of the same type and the conveying defect of the bank note.
A bank note counter for counting, e.g., a plurality of bank notes of the same type, counts the notes passing one by one before a detector, and simultaneously detects the defect of the note itself (e.g., a mixture of a bank note of different type, a contamination, a deformation, and the turning of the note inside out) as well as the conveying defect of the note (e.g., the posture of the note, feeding of superposed notes, or continuous notes), thereby stopping the note counting operation of the counter when the defect is detected. As a conventional method of detecting such defect, there is, for example, a method comprising the steps of setting and storing as set values the printing pattern of a reference bank note preselected for each type of notes in a memory, and comparing the printing pattern signal of the detected note from the detector with the set printing pattern value signal from the memory. The detector operates, for example in a light transmission type, to emit a light from a light source to a bank note to be detected and to receive the light passed through the note by a photodetector, thereby producing the pattern signal of the bank note.
However, when the defect of the note is thus discriminated, the normal note may be erroneously discriminated as the defective note. Because paper powder, dusts on the note and printing ink adhere the detector, and because the detecting sensitivity of the detector is gradually deteriorated according to the aging decrease in the luminous intensity of the light source, the difference between the printing pattern of the reference note set and stored in the memory and the printing pattern detected by the detector gradually becomes large.
Therefore, it has been desired to provide a method of discriminating a sheet which can eliminate the above mentioned disadvantages and hence, which is not affected by the influence of the aging of a detector.